PROJECT SUMMARY/ABSTRACT The loss of skeletal muscle mass is characteristic of inactivity-, disease-, and age-related conditions and negatively affects whole muscle function and quality of life. Consequently, understanding the mechanisms of muscle fiber size maintenance and growth is important for guiding interventions and therapeutics. In response to short-term loading (two weeks), muscle stem cells (satellite cells) are not necessary for hypertrophy in adult mice. However, after long-term loading (eight weeks), the absence of satellite cells results in robust expansion of the cytosolic area that myonuclei govern (myonuclear domain), excessive extracellular matrix deposition, and blunted muscle fiber growth. Whether compromised growth is a consequence of fibrosis or a lack of myonuclear addition mediated by the loss of satellite cells is presently uncertain. To understand the role of satellite cells during prolonged muscle hypertrophy, our laboratory recently developed the Pax7-N-WASp mouse. Inducible depletion of N-WASp specifically in satellite cells inhibits their ability to fuse into a muscle fiber and contribute a nucleus during hypertrophy. However, their presence and activation during growth can regulate extracellular matrix accumulation. This mouse model will elucidate whether limiting fibrosis promotes muscle fiber growth in the absence of myonuclear accretion. Furthermore, the ability to prevent satellite cell fusion in vivo will allow for exploration into whether satellite cells communicate with muscle fibers in the absence of fusion. The findings from this proposal will provide fundamental information on the necessity of satellite cells during adult skeletal muscle hypertrophy and potentially define new roles for satellite cells independent from fusion into muscle fibers.